In heterodyne interferometers, coherent radiation components having slightly different frequencies overlap. A signal having the difference frequency of the two radiation components, known as heterodyne frequency, is then obtained at the detector device. The information, for example, about the shape or roughness of surfaces, is contained in the phase angle of the detected frequency.
Coherent radiation components are obtained by using a shared light source, whose radiation is split into two partial beams with the aid of a beam splitter, for example. Changing the frequency in at least one partial beam using acousto-optical modulators is performed for obtaining different radiation frequencies in the two partial beams. A frequency shift in the range of a few tens of MHz, typically on the order of 35 MHz, depending on the selected control frequency, is then achieved using an acousto-optical modulator. If an acousto-optical modulator is provided for only one partial beam, the control frequency corresponds to the heterodyne frequency obtained at the detector device. This is the reason for one disadvantage of this design, because for the same frequencies of the electrical control for the acousto-optical modulator and the analysis in the detector device, interference with the useful signal in the detector by the control signal of the modulator is difficult to prevent. The high analysis frequency represents another disadvantage. Such high frequencies require a relatively high degree of complexity to minimize crosstalk, noise, and reflections.
It is therefore advantageous to provide one acousto-optical modulator having a slightly different control frequency for each partial beam. The heterodyne frequency is given by the difference between the control frequencies and may be specified accordingly. Therefore the analysis frequency of the detector device is different from the control frequencies of the acousto-optical modulators, whereby interference with the detector signals may be largely avoided. At the same time, a relatively low heterodyne frequency results, mostly on the order of a few hundreds of kHz, which makes simple analysis of the phase difference possible for calculating the surface geometry to be measured. The disadvantage of this design is that two acousto-optical modulators must be provided, which results in increased costs.
Heterodyne interferometers may be designed as a Mach-Zehnder system or as a Michelson interferometer. The Mach-Zehnder system offers the advantage that a partial beam passes through the acousto-optical modulator(s) only once. In the case of a Michelson interferometer, the partial beam passes through the acousto-optical modulator twice. This makes it more difficult to influence the partial beam using the acousto-optical modulator.